The present invention relates to an optical disk apparatus for recording and reproducing data on and from an optical disk. More particularly, the invention relates to an optical disk apparatus which can correct a focus value accurately within a short time and which can thus have good recording/reproducing characteristic. The invention also relates to a focus-value correcting method that can correct a focus value accurately within a short time. The invention further relates to an optical disk.
In the field of optical disk apparatuses for recording and reproducing data on and from optical disks, compact disks (CDs) having audio signals recorded on them have already been in use in large numbers. Recently, digital versatile disks (DVDs), which have a large storage capacity and from which data can be transferred at high rate, have come into use as media for recording motion pictures. In addition, MO (Magneto-Optical) disks, PDs (Phase-Change Disks) and DVD-RAMs have been put to practical use as recording/reproducing disks.
Thanks to the advanced microprocessor technology and the like, not only text data, but also audio data, still-picture data, motion-picture data and the like can be recorded on and reproduced from the above-mentioned various kinds of disks. It is therefore increasingly demanded that disk systems be developed which have a storage capacity large enough to store these various items of data and which can change the rate of transferring the data.
To increase the data storage capacity of the optical disk system, it is generally necessary to reduce the size of the beam spot in the operation of recording and reproducing data on and from the disk, thereby to record data at high density on the optical disk. The size a of a beam spot is given as follows:a=α×λ/NAwhere α is the constant determined by the distribution of light bean intensity, λ is the wavelength of the light beam, and NA is the numerical aperture of the objective lens provided in the optical disk system.
Thus, a small beam spot can be formed on the surface of the optical disk if the optical disk system has a semiconductor laser for emitting a beam of short wavelength and an objective lens having a large NA value. If so, the system can record data at high density on the optical disk and can thereby have a large storage capacity.
If the beam spot is made small by this method, however, the focal depth will decrease. In other words, the tolerance for defocusing (i.e., focusing error) will inevitably decrease. The focal depth d is usually expressed as follows:d∝λ/(NA)2.
That is, the shorter the wavelength of the light beam and the larger the NA, the more the focal depth will decrease. In the optical disk apparatus for recording and reproducing data on and from an optical disk at high density, high-precision focusing control should, therefore, be performed to place the recording surface of the disk within the focal depth of the objective lens.
The defocusing occurs in the focusing control, due to various causes. It may result from the steady-state deviation that has not been eliminated completely during the servo control. It may result from the difference between optical disks in terms of substrate thickness. It may be caused by the changes in the servo target value, due to the offset changes that occur as the temperature rises in the optical disk apparatus.
Of these causes of defocusing, the steady-state deviation is determined by the size and gain of the focus-disturbing element. In most cases of high-density, data-recording/reproducing, the disturbance (e.g., warping of the disk) are reduced and the gain of the focus-disturbing element are increased, thereby minimizing the steady-state deviation. To reduce the defocusing resulting from the difference between optical disks in substrate thickness, thereby to raise the focusing precision, the focus may be automatically adjusted in accordance with the jitter value or the like of the ROM section. In this case, it suffices to change the focus bias to various values and select the most desirable one in accordance with the jitter value detected at the time of inserting the disk is inserted into the optical disk apparatus.
While the optical disk apparatus is operating, it is difficult to eliminate the offset change or the like resulting from the temperature rise in the optical disk apparatus. To eliminate the offset change or the like during the operation of the optical disk apparatus, the change of the offset must be detected at short intervals by simple means, thereby to move the focus to an optimal point. In practice, however, the change of the offset can hardly be detected at short intervals by simple means during the operation of the optical disk apparatus.